This invention relates to a measurement apparatus and a method of measuring the gas permeability of rock and other geological materials. In particular, though not exclusively, the invention is for use in the petroleum, geotechnical, built environment and groundwater industries.
The measurement of rock permeability, porosity and fluid saturation from core samples in oil and gas exploration work gives vital information of the state of the reservoir under consideration, its potential flow capacity and can also provide insight into the most efficient methods of oil and/or gas extraction. Taking measurements of the gas permeability of rocks provides a method of classification of the rock type and quality. Permeability data are used both as absolute values and to provide correlation statistics.
At present, permeability has been measured utilising either a core plug drilled from the rock sample at predetermined intervals, or by a contact method. For contact methods a probe is normally used. The probe is commonly in the form of a pipe having an orifice of known diameter at one end, with a sealing ring at this end for forming a seal with the surface with which the probe end makes contact. In use, the probe end is placed in direct contact with the rock whose permeability is to be measured. A fixed rate flow of gas is then flowed through the pipe (at either a predetermined or continuously varying pressure). A pressure drop is caused by gas entering the rock, the pressure drop being dependent on the, permeability of the rock.
Point sampled measurements of this pressure drop can be used to obtain a measure of the local permeability of the rock material.
The main problem with such contact measurement methods is the errors produced by variation of the coupling of the probe to the rock, and statistical errors induced by an inappropriate sampling regime when the natural heterogeneity of the rock material and/or its surface is considered. Both the volume of rock being investigated, its relationship to the theoretical statistical support volume and the restrictive frequency of the point sampling nature of these experiments, lead to potential aliasing of the frequency of natural geological variability, and have thus inhibited the utility and application of the method. With core plug sampling these potential errors are compounded by the enforcing of a strict one-dimensional pressure field upon the sample. The currently used methods are also very time consuming since the probe needs to be lifted up and down (i.e. out of contact with the sample and back into contact with the sample) between each measurement.
The present invention seeks to avoid or minimise one or more of the foregoing disadvantages. In particular, the invention seeks to increase accuracy of the measurement of the permeability, while also speeding up data acquisition.
Accordingly, the present invention provides an apparatus suitable for use in measuring permeability of a material, the apparatus comprising: a non-contact probe comprising a first conduit and a second conduit which are arranged so that their open ends are contiguous; a gas inlet for admitting a flow of gas into a first space inside said first conduit; and a pressure difference measuring system for measuring a pressure difference between said first space and a second space inside said second conduit; wherein there is provided a probe support formed and arranged for supporting the open end of the probe at a predetermined height above a surface of said material, in use of the apparatus; and wherein said apparatus includes processor means programmed for converting a said pressure difference measurement into a permeability value, in use of the apparatus. Normally the second conduit would be blind i.e. the end remote from its open end, would be closed.
Various different forms of conduits may be used in the apparatus of the invention. Advantageously, the conduits are configured so as to provide a relatively large degree of contiguity therebetween. Conveniently the conduits are configured so that one of said conduits extends along opposite sides of the other conduit. Most conveniently one of said conduits surrounds the other conduit, said conduits being defined by substantially coaxial pipes. Other possible configurations comprise coaxial elongate, generally slot-form, rectangular cross-section pipes. Where coaxial pipes are used, it will be appreciated that either the outer or the inner conduit may be used for the gas supply.
In a particularly simple form of the invention, the probe support comprises a spacer element projecting below the open end of the probe whereby said open end may be held above the material surface by contacting the spacer with the material surface.
Advantageously the probe is mounted on a moveable head assembly and the apparatus further includes a position control system formed and arranged for controlling the movement of the head assembly so as to cause the probe to scan across a surface of the material to be analysed, in use of the apparatus, while maintaining the probe at a constant distance from said surface, whereby pressure difference measurements from a plurality of points across said surface, using said pressure difference measuring system, may be readily collected by scanning of said surface by said probe.
Thus, in a preferred aspect the present invention provides an apparatus suitable for use in measuring permeability of a material, the apparatus comprising: a non-contact probe comprising an inner pipe and an outer pipe which are arranged coaxially; a gas inlet for admitting a flow of gas into a first space defined between the inner and outer pipes; and a pressure difference measuring system for measuring a pressure difference between said first space and a second space comprising the interior of the inner pipe; wherein the probe is mounted on a moveable head assembly and the apparatus further includes a position control system formed and arranged for controlling the movement of the head assembly so as to cause the probe to scan across a surface of the material to be analysed, in use of the apparatus, while maintaining the probe at a constant distance from said surface, for collecting pressure difference measurements from a plurality of points across said surface, using said pressure difference measuring system, during scanning of said surface by said probe.
The control means preferably includes a distance-measuring means i.e. rangefinder device for measuring, preferably substantially continuously in time, the separation between the probe and the surface of the material being analysed i.e. the height of the probe above the surface. The distance measuring means preferably provides a feedback signal, representing the amount of any change in said measured distance, to the control means which is adapted to control the spacing of the head assembly from the surface of said material so as to maintain the probe at a constant height above the surface of said material. Preferably, the distance-measuring means is a distance-measuring laser.
In general the apparatus is formed and arranged so that the probe is supported at from 0.1 to 0.5 mm above the material surface, advantageously from 0.2 to 0.3 mm above the surface. It is important that the probe end is supported at a constant height above the material surface. Preferably the height should be kept with +/xe2x88x9210%, most preferably +/xe2x88x925%, desirably +/xe2x88x923% of a predetermined height. It is generally preferred though to limit the size of the probe and diameter in order to maintain a reasonable spatial resolution for permeability measurements across the surface of the material. Typically there is used an overall probe end diameter in the range from 2 to 5 mm.
The apparatus may include a data processing device for calculating a value representing the permeability of said material to said gas, for each collected pressure difference measurement, and preferably immediately following collection of that pressure difference measurement. Alternatively said values representing permeability may be calculated for a multiplicity of pressure difference measurements subsequent to the collection of all of said multiplicity of pressure difference measurements.
The apparatus preferably further includes a memory or other storage device for storing the collected pressure difference measurements and/or the calculated values representing permeability, together with respective surface position information associated therewith. This enables the variation in permeability across the area of the surface of the material to be subsequently mapped out. The apparatus may conveniently include a computer device programmed so as to generate a map of the permeability and for displaying said map on a visual display unit which may be connected to the processor in use of the apparatus.
Preferably, the position control system comprises at least one positioning device, the or each said positioning device comprising at least one of a precision linear-scale slide and a stepper motor, with an associated control device therefor.
As noted above, a particular benefit of the invention is the contactless nature of the probe used and the speed of operation of the apparatus. These benefits can advantageously be significantly extended by providing at least one contactless measuring device on the moveable head assembly of the apparatus, for making measurements of other material properties at the same time. Thus, for example, the apparatus may also include at least one laser for use in making acoustic property measurements for use in determining physical properties such as Young""s modulus. In this approach the surface is subjected to a high-energy laser burst, which creates a shock wave in the material. The amplitude and velocity of this shock-wave can be sensed utilising a laser (preferably a second laser) monitoring the surface wave""s amplitude by either reflection triangulation (which is described in further detail later) or, preferably, by interferometry of the light wavelength.
Preferably, the apparatus may also include at least one laser measuring device formed and arranged for measuring the height of a point on said surface relative to a predetermined datum, for use in making surface roughness measurements.
Surface roughness measurements can be of particular value in the context of the present invention, since the flow of fluid through rock strata is, in practice, usually dependent on both the permeability of the material and the conductivity of the material which is flow or transport of fluid through cracks in the material. The latter is increasingly dependent on the surface roughness of the opposed faces of the material which define a crack in the material, as the thickness of the crack reduces.
Preferably, the apparatus may also include at least one laser formed and arranged for volatilising material from a point on said surface, and a spectrometer associated therewith, for carrying out analysis of the constituent components of the material being analysed. Preferably, the spectrometer is a mass spectrometer or an infrared spectrometer.
In a further aspect the present invention provides a method of measuring permeability of a material, comprising the steps of:
a) providing an apparatus according to claim 1;
b) passing gas through said first space towards the surface of said material, at a predetermined flowrate;
c) supporting the probe end at a fixed distance from the surface of the material;
d) measuring a pressure difference between said first space and said second space so as to obtain a pressure difference measurement; and
e) determining a permeability value for the pressure difference measurement.
According to a second preferred aspect of the invention, there is provided a method of measuring permeability of a material, comprising the steps of:
a) providing a probe comprising an inner pipe and an outer pipe arranged coaxially, and positioning the probe substantially perpendicularly to a surface of a material to be analysed;
b) passing gas through a first space defined between the inner and outer coaxial pipes, towards the surface of said material, at a predetermined flowrate;
c) scanning the probe across the surface of said material, while also maintaining the probe at a fixed distance from the surface of the material;
d) measuring a pressure difference between said first space defined between said inner and outer pipes, and a second space comprising the interior of said inner pipe, at a plurality of different points across said surface of the material; and
e) determining a permeability value for the pressure difference measurement at each said point.
The pressure difference measurements are preferably collected at lateral distance intervals of from a minimum of about 20 xcexcm to any user defined maximum interval, during the lateral movement of the probe.
Alternatively, the pressure difference measurements could be collected at predetermined time intervals, for example every 0.1 seconds, during the lateral scanning movement of the probe at a predetermined rate across the material surface, typically there may be used a scanning speed from 0.1 to 10 cm secxe2x88x921, conveniently about 1 cm secxe2x88x921.
Preferably, the method further comprises the step of
f) measuring one or more other properties of the material using one or more devices selected from the group of: a laser with sensor for making surface roughness measurements; at least one laser for making acoustic property measurements; and a laser and a spectrometer for analysing constituent components of the material.
Any suitable form of sample having generally level surface may be used in the method of the invention. Conveniently though there is used a longitudinally sliced slab from a cylindrical core sample. Typically such a slab might be 20xc3x978xc3x971 cms in size.